In an optical communication system, light having a predetermined wavelength is modulated with signals to be propagated through a transmission optical fiber. Recently attention has been paid to a wavelength multiplexed communication system in which lights having different wavelengths are propagated to enhance the density of signals and the transmission speed. In such a communication system, lights having different wavelengths are radiated from light emitting devices, and the lights which are multiplexed in an optical coupler are demultiplexed and detected in a photo-detecting device.
As a photo-detecting device which is applied to such an optical communication system, a PIN type photo-detector is known which includes an optical absorption layer of In.sub.0.53 Ga.sub.0.47 As having lattice matching on an InP substrate as described on pages 653 and 654 of "Electron. Lett. 1984, Vol. 20". Also known is an avalanche multiplication type photo-detector as described on pages 257 and 258 of "IEEE, Electron Device Lett. 1986, Vol. 7". The PIN type photo-detector has an advantage in that its capacitance value is small and the fabrication process thereof is easy; the avalanche multiplication type photo-detector has an advantage in that the internal gain effect is expected in it and the high speed response is obtained.
In operation, when light having an energy greater than a band energy of the optical absorption layer is supplied to a light receiving portion to be absorbed therein, electrons and holes are induced therein so that the transit of carriers is realized to produce an electric current signal in accordance with the application of a reverse bias.
However, there is a disadvantage in the above described photo-detectors in that it is difficult to demultiplex and detect optical multiplexed signals having different wavelengths for the structural reason that the photo-detector is used in the form of one chip device. This is because it is not easy in the structure of the photo-detector to discriminate the electric current produced therein in accordance with its wavelength.
In such a situation, a two-wavelength demultiplexing superlattice PIN waveguide detector in which two lights having different wavelengths are demultiplexed and detected has been proposed by A. Larsson et al as described on pages 233 to 235 of Appl. Phys. Lett. 49(5), 4 Aug. 1986". The two-wavelength demultiplexing superlattice PIN waveguide detector comprises a substrate of n-GaAs, and an n-AlGaAs layer, a superlattice structure consisting of n-GaAs layers/n-AlGaAs layers, a p-AlGaAs layer, and a p-GaAs layer grown successively on the substrate wherein two waveguide regions are defined in the p-AlGaAs and GaAs layers in accordance with the formation of the proton implantation region, which electrical resistance is increased by the proton implantation at a region other than the waveguide regions, and further comprises two electrodes for applying different voltages to the two waveguide regions separately, and an electrode provided on the bottom surface of the substrate.
In operation, the lights having two different wavelengths is supplied to the PIN waveguide detector. When one of the lights s absorbed in the waveguide layer, the light is converted to electric current. In this stage, whether the light is absorbed therein or is waveguided therethrough is determined dependent on the absorption edge energy of the waveguide layer. That is, if the light is of an energy larger than the absorption edge energy, in other words, if the light is on the short wavelength side, the light is absorbed. On the other hand, if the light is of an energy lower than the absorption edge energy (on the long wavelength side), the light is waveguided through the waveguide layer.
The PIN waveguide detector is characterized in that the layer for an optical absorption and waveguide has a superlattice structure including wells and barriers. In this regard, an absorption edge energy of the superlattice structure is different from that of a widely used bulk structure. In more detail, the absorption edge energy E of the superlattice structure is greater in accordance with the quantum level of holes and electrons in the superlattice structure than the absorption edge energy E.sub.g of the bulk structure. This is expressed in the equation (1). ##EQU1## where l is a thickness of well layers, E.sub.g is a band energy for a bulk structure, m.sub.n is a mass of electron, m.sub.p is a mass of hole, and h is defined as ##EQU2## where h is a Planck constant.
Here, it is assumed that an electric field is applied across the superlattice. This results in the spatial separation between electrons and holes in a well layer. As a result, a phenomenon called "Stark effect" in which the band energy becomes lower when an electric field applied than when no electric field applied is observed. The absorption edge energy is largely varied dependent on the applied electric field whereby the wavelength demultiplexing and detection is realized in the vicinity of an absorption edge.
According to the above described two-wavelength demultiplexing superlattice PIN waveguide detector, however, there are following disadvantages.
First, the formation of the high-resistance region is not easily realized by the proton implantation. For this reason, electrical crosstalk is inevitably induced between the waveguide regions. Furthermore, the proton implanation is liable to cause the introduction of defects into the detector. Such defects must be absolutely avoided in an optical communication device in which reliability is regarded the most important requirement.
Second optical confinement is not efficiently realized in a coupling region between the waveguide regions. This causes a coupling loss of waveguided lights. For the purpose of reducing the coupling loss, it is considered that the distance between the waveguide regions is shortened. However, this causes the increase of the aforementioned electrical crosstalk.